Background

Patient-ventilator dyssynchrony is frequently observed during assisted mechanical ventilation (MV). However, the effects of expiratory muscle contraction on patient-ventilator interaction are underexplored. We hypothesized that active expiration would affect patient-ventilator interaction and we tested our hypothesis in a mixed cohort of invasively ventilated patients with spontaneous breathing activity.

Methods

This is a retrospective observational study involving patients on assisted MV who had their esophageal (Pes) and gastric (Pgas) pressures monitored for clinical purposes. Active expiration was defined as Pgas rise (ΔPgas) ≥1.0 cmH2O during expiratory flow without a corresponding change in diaphragmatic pressure (Pdi). Waveforms of Pes, Pgas, Pdi, flow, and airway pressure (Paw) were analyzed to identify and characterize abnormal patient-ventilator interaction.

Results

We identified 76 patients with Pes and Pgas recordings, of whom 58 demonstrated active expiration with a median ΔPgas of 3.4 cmH2O (IQR=2.4-5.3) observed in this subgroup. Among these 58 patients, 23 presented the following events associated with expiratory muscle activity: (1) distortions in Paw and flow that resembled ineffective efforts, (2) distortions similar to autotriggering, (3) multiple triggering, (4) prolonged ventilatory cycles with biphasic inspiratory flow, with a median % (IQR) increase in mechanical inflation time and tidal volume of 54% (44-70%) and 25% (8-35%), respectively and (5) breathing exclusively by expiratory muscle relaxation. Gastric pressure monitoring was required to identify the association of active expiration with these events. Respiratory drive, assessed by the rate of inspiratory Pes decrease, was significantly higher in patients with active expiration (median [IQR] dPes/dt: 12.7 [9.0-18.5] vs 9.2 [6.8-14.2] cmH2O/sec; p<0.05).

Conclusions

Active expiration can impair patient-ventilator interaction in critically ill patients. Without documenting Pgas, abnormal patient-ventilator interaction associated with expiratory muscle contraction may be mistakenly attributed to a mismatch between the patient´s inspiratory effort and mechanical inflation. This misinterpretation could potentially influence decisions regarding clinical management.

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